An FPGA-based multi-frequency EIT system with reference signal measurement

Abstract

Multi-frequency electrical impedance tomography (MFEIT) has been rapidly developed, from theory to application, in the biomedical imaging. Compared with the traditional electrical impedance tomography (EIT), MFEIT provided more information for physiological status analysis or pathological diagnosis. In this work, a MFEIT system based on FPGA is designed, which includes the following new characteristics. First, a high-accuracy multi-frequency current source with adjustable output frequency and amplitude as well as high output impedance is designed based on the combination of a classic current mirror and a differential topology, which demonstrates stable performance in high frequencies. Second, a reference measurement circuit is developed for the calibration of phase measurement data, through which the amplitude data and phase data can be more accurately measured. Furthermore, the low-pass filters of system have been designed based on the Butterworth filter theory to realize the flattest frequency response in the passband. A series of systematic tests are performed on the MFEIT system, including filter testing, amplitude and phase signal-to-noise ratio (SNR) measurements, channel consistency and measurement repeatability verification. Finally, the flatness analysis of the system frequency response and the load dependency discussion are performed respectively. In all, the maximum amplitude SNR and phase SNR of the boundary measurements are 71.55dB and 64.12dB respectively. The frequency response curve at low loads is almost flat across the entire bandwidth between 20Hz and 1MHz. Although the frequency response deteriorates when the magnitude of load is as high as kΩ, it remains flat in the frequency range of 200Hz-100kHz. The evaluation results indicate that the system has good performance over a wide frequency band, and achieves a relatively high SNR.